This invention relates to a method of producing partially neutralized (meth)acrylic acid polymers excellent in water absorbing capacity.
Recently, water-absorbing resins are used very widely, for example in the field of hygiene (in sanitary napkins, tampons, diapers, disposable dusters, etc.), in the field of agriculture/horticulture (as water retaining agents), and in other fields (for coagulation of mud and sludge, prevention of dew condensation on building materials, dehydration of oils, etc.). Known water-absorbing resins include, among others, partially polybasic acid-crosslinked carboxymethyl cellulose, partially crosslinked polyethylene oxide, hydrolyzed starch-acrylonitrile graft polymers and partially crosslinked polyacrylic acid salts.
Among such resins, hydrolyzed starch-acrylonitrile graft polymers and partially crosslinked polyacrylic acid salts are particularly of practical use since they have relatively high water absorbing capacity.
Other noteworthy known ones are highly water-absorbing resins obtained by reversed-phase suspension polymerization of partially neutralized (meth)acrylic acid, namely a mixture of (meth)acrylic acid and a (meth)acrylic acid salt, if necessary in the presence of a protective colloid such as sorbitan fatty acid ester, cellulose ether, cellulose ester, polyvinyl alcohol or polyethylene oxide (cf. Japanese Kokai Tokkyo Koho Nos. 44627/1982, 62665/1984 and 186506/1985).
The term "reversed-phase suspension polymerization" as used herein means the polymerization of a monomer in the form of an aqueous solution suspended in a non-aqueous solvent.
Meanwhile, it is not the only requirement for highly water-absorbing resins that they should have a high degree of absorbency for water. It is also important that they should show a sufficiently high rate of water absorption upon coming into contact with water, namely a capacity for absorbing a sufficient quantity of water at a sufficient rate to render them of practical value, and at the same time have the so-called insolubility, namely the property of being insoluble when brought into contact with water.
However, none of the known absorbent resins has the above performance characteristics in a well-balanced manner. Solution of such problem will necessarily lead to an increase in the range of use of highly absorbent resins, hence is very much desired.